My 5 mth old pup has been very quiet since last night he had a bit of watery sick a day or so ago but it was one very tiny bit so i didn't worry too much (although kept an eye on him)but since this morning he's just slept and hasn't eaten anything all day,(doesn't seem in pain or uncomfortable)he normally follows me around but hasn't bothered,preferring to kip on the couch,he did get up willingly to go for a walk but wasn't quite as lively as usual and seemed to feel the cold a bit.He's been in perfect health since i got him,eats a good diet,has plenty of walks,is fully vaccinated and wormed so i've been racking my brains to think if he's eaten anything bad etc and can't think of anything although we went for a walk to a country park on sat and he went to lap at the waters edge but i pulled him away as it had alot of algae in it so if he has drunk any it would be tiny.I don't want to overreact but he's not his usual self and i'm starting to really worry(love him so dearly)might get him to vets tonight.what do you think.

If you're at all worried, take him to the vet. It's possible he's eaten/drunk something that didn't agree with him. You don't say what breed he is, he could just be having what we used to call "growing days" when our big lad did nothing but sleep (and grow)! When he 1st started doing it, I thought he was ill but we came to the conclusion he just had days where he wanted to sleep.
Hope he's OK.